This investigation is concerned with the cellular mechanisms underlying intestnal transport of water, inorgaaic ions, and water soluble non-eectrolytes, e.g., sugars and amino acids. Isolated bullfrog small intestine is being used as an exprimental system. Specific studies in progress include: (1) Investigation of the metabolic dependence (e.g., sensitivity to inhibitors, substrate requirements etc.) of net transepithelial Na and Cl transport by this tissue. (2) A study of the coupling between the transport of Na (and K) and that of water, sugars and amino acids. (3) Measurement of the response of transmural water and electrolyte movements to externally applied electrical and osmotic gradients. (4) Measurement of intraepithelial Na, K, and Cl activities and the effect of transported solutes on these parameters. Experimental methods involved include short circuit, voltage clamp, and radiotracer techniues; chemica determination of intracellular water Na, K and Cl; measurement of mucosal and serosal epithelial cell membrane potentials, and the use of ion selective microelectrodes to determine intracellular ionic activities.